In the related art, a base station in a mobile communication system includes a transceiving filter device having an amplification circuit between a tower antenna and a transceiving circuit for amplifying a received signal.
For example, Patent Document 1 (International Publication No. 02/31997 pamphlet) shows a transceiving filter device having first and second transmission paths between a first port connected to an antenna and a second port connected to a transceiving circuit.
This structure is shown in FIG. 8. In FIG. 8, reference symbol ANT denotes an antenna, reference numeral 10 denotes a transceiving circuit, reference numeral P1 denotes a first transmission path that allows a received signal to pass, and reference numeral P2 denotes a second transmission path that allows a transmission signal to pass. The first transmission path P1 includes receiving filters Rx1 and Rx2 having a bandpass characteristic that allows a received signal to pass, and an amplification circuit LNA. The second transmission path P2 includes a transmitting filter Tx1 having a bandstop characteristic that stops only a received signal.
With this structure, a received signal input from the antenna ANT is amplified in the first transmission path P1, and is then sent to the transceiving circuit 10, and a transmission signal from the transceiving circuit 10 is sent to the antenna ANT via the second transmission path P2.
However, as shown in FIG. 8, when the first and second transmission paths P1 and P2 are provided and either path includes an amplification circuit, the two transmission paths may form a feedback loop. The transmitting filter Tx1 passes the transmission frequency band and stops the receiving frequency band. However, if the amount of attenuation in the vicinity of the receiving frequency band of the transmission filter is not satisfactory, the output signal of the amplification circuit LNA is oscillated because of positive feedback in the path of Rx2→Tx1→Rx1→LNA. In order to suppress occurrence of such oscillation, a large amount of attenuation in the receiving frequency band of the transmitting filter Tx1 must be maintained so that the loop gain is equal to or less than 1. Therefore, it is necessary to increase the stop bandwidth of the receiving frequency band of the transmitting filter Tx1 or to reduce the pass bandwidths of the receiving filters Rx1 and Rx2. As a result, there arises a problem in that insertion loss (IL), nose figure (NF), and group delay (GD) characteristics in the transmission frequency band or the receiving frequency band are degraded.
Accordingly, it is an object of the present invention to provide a transceiving filter device that suppresses degradation in the insertion loss, noise figure, and group delay characteristics in the transmission frequency band or the receiving frequency band and that prevents oscillation, and to provide a communication apparatus using the transceiving filter device.